(1) Field of the Invention
The invention relates to a regulated battery charge current circuit, and more particularly, to a regulated charge current circuit having a digitally controlled low-pass-filter within the feedback loop for optimized loop stability and simple adjustment.
(2) Description of the Prior Art
Regulated charge current circuits use an analog low pass filter in their feedback loop, which require certain precautions to achieve sufficient stability. With state-of-the-Art designs, the feedback loop is typically realized by an analog amplifier followed by an analog integrator.
In order to get stable operation, the dominant pole of the regulator function needs to be selected at a very low frequency. The disadvantage of a low dominant pole is that the regulator becomes slow. This is a problem if the charge current is switched on and off at a higher frequency than this dominant pole (e.g. in case of a pulsed charge current or a rectified sine wave power supply). Then the regulator won""t be able to reach the stable state within one pulse and the charge current will be too low. Further, the problem with this type of design is that the capacitors have to be very large.
FIG. 1 shows a typical state-of-the-art charge current regulator circuit design. The transistor N11, connected to the supply source US1 and operating as a current valve, feeds the charge current IC1 to the battery BAT1. Said charge current IC1 produces a drop voltage across the shunt resistor RS1. A voltage reference U-REF1, which is equivalent to the desired charge current, multiplied by the shunt resistor RS1, represents the current reference. The amplifier AMP1 typically has a well-defined gain and amplifies the voltage difference across said shunt resistor RS1 and the voltage reference U-REF1. Its output voltage is integrated by the following low pass filter LPF1. When said charge current is below the desired value, said analog low-pass filter LPF1, will ramp up to increase the control current I-CTL1, which will then further open said current valve N11. When said charge current is above the desired value, said analog low-pass filter LPF1 will then ramp down to decrease the control current I-CTL1, which will then begin to close said current valve N11. Transistor N12 works to enable or disable the battery charging process.
As stated before, state-of-the-art regulated charge current circuits typically use an analog low pass filter in their feedback loop, which require certain precautions to achieve sufficient stability. Said feedback loop could also be realized by an analog sample-and-hold-circuit followed by an analog integrator; in this case the comparator would have to be an amplifier with a fixed gain. The problem is that the capacitor in the low-pass filter would have to be very large due to leakage currents.
Several patents describe circuits for charge current regulators.
U.S. Pat. No. 6,177,787 (to Stephen W. Hobrecht) discloses circuits and methods for controlling timing and slope compensation in switching regulators. These circuits and methods include a timing control circuit that controls the timing of the switching of one or more switching regulator output stages so that the switching occurs at evenly spaced time intervals, and a slope compensation circuit that produces a slope compensation signal having a waveform that need not match the waveform of any oscillator signal, nor that need have the same period as the oscillator signal. Slope compensation is provided by generating a slope compensation signal using decoding logic, a digital-to-analog converter (DAC), and an integrator.
U.S. Pat. No. 5,969,515 (to John W. Oglesbee) describes a digital control circuit used for controlling the load current of a power converter circuit, and comprising a reference circuit, analog comparator circuit, control logic circuit, and a counter circuit. The control logic circuit controls the operation of a power switch, and uses counters in conjunction with the analog comparator circuit and reference circuit, to determine when to open and close the power switch. The load current is allowed to vary between a first and second preselected load current level.
A principal objective of the invention is to achieve improved loop stability for a charge current regulator, especially in the environment of pulsed supply voltage. This is done by replacing the analog feedback circuit with its inherent functional tolerances with a more precise digital feedback circuit. In addition, it should be possible to change the circuit""s timing characteristics in a wide range during the circuit""s normal operation.
Another major objective is to regulate a charge current from a pulsed power supply, such as a rectified sine wave power supply without a large filter capacitor. An additional objective for this regulator is to provide a pulse-width-modulated charge current.
One key objective is the capability to freeze the regulator state while the regulator is temporarily disabled, e.g. when the supply voltage is too low, and then to resume with no recovery time with the last defined regulator value.
A further objective is to start the circuit operation with an always well-defined circuit state.
A further objective is to reduce the chip area required, compared to an analog circuit, requiring large capacitances.
In accordance with the objectives of this invention, a circuit to achieve improved loop stability for a charge current regulator, especially in the environment of pulsed current sources, is achieved. The circuit comprises a device to regulate a charge current, operating as an electric current valve, a shunt resistor, where the charge current produces a voltage proportional to said charge current and an adjustable reference voltage source, which represents the desired charge current. The circuit also contains a comparator circuit to compare said voltages across said shunt resistor and said reference voltage.
Key element of this invention is a digital low pass filter within the feedback loop, formed by an up/down counter, a variable frequency oscillator and a digital-to-analog-converter. For this purpose the circuit provides an up/down counter, counting up or down depending on said comparator""s result. Said up/down counter can be preset to a variety of specific digital values. Further the circuit provides a variable frequency oscillator to step said up/down counter in a regular time interval. Said oscillator can be set to a variety of frequency values and the circuit""s control logic can enable/disable said oscillator. Furthermore the circuit contains a digital-to-analog-converter, producing an output signal proportional to the up/down counter""s digital value. Said digital-to-analog-converter""s output signal will then regulate said current regulating device. The circuit""s control logic can enable/disable said current regulating element for the purpose of a pulse-width-modulated (PWM) charge current. When it is disabled (due to a too low input voltage or when signal EN2=low) the regulation loop is suspended and the state of the up/down counter will be stored until the element is enabled again. Therefore the regulation of the charge current will only take place as long as it is switched on. That""s why the duty cycle of the pulse-width-modulated battery charge current, controlled by the control signal EN2, will not affect the regulation loop even if the regulator is much slower than the frequency of the PWM signal.
As said before, the key element of this invention, to achieve the objectives, is the digital low pass filter, built by the up/down counter, the digital-to-analog-converter and the variable frequency oscillator. The timing characteristics of such circuit are significantly different from state of the art analog solutions, thus avoiding many of the problems of state-of-the-art solutions. Furthermore, the circuit is capable of regulating a charge current from a pulsed power supply, such as a rectified sine wave power supply without a large filter capacitor and can provide a pulse-width-modulated charge current.
One key feature is the capability to freeze the regulator state while the regulator is temporarily disabled, e.g. when the supply voltage is too low, and then to resume with no recovery time with the last defined regulator value. The circuit also allows to switch the battery charging current off, when the supply voltage is below a specified minimum, switch said battery-charging current on, when the supply voltage passes said specified minimum and resume regulation at the previously saved value.
Furthermore the regulator according to this invention can provide a pulse-width-modulated battery charge current, controlled by an external pulsed control signal, while it maintains stable current regulating characteristics. While the battery charge current is switched on, said digital controlled charge current regulator performs its normal regulating function. While the battery charge current is switched off, said digital controlled charge current regulator is halted: it freezes said up/down counter, thus keeping the last digital counter value. When said battery charge current is switched on again, the regulator resumes said up/down counter operation and the battery charge current resumes at the last saved value, without requiring any extra recovery time.